Broaching workpieces to cut a circular array of slots therein to provide teeth between adjacent slots is an efficient operation to produce accurate gears or splines, either internally or externally toothed. In the operation as known to the prior art, the cutting teeth of the broach were usually arranged in longitudinally extending series and in axially spaced, circumferentially extending sets.
The broach may be an internally toothed pot broach for cutting slots to form external teeth on the periphery of a circular workpiece. Alternatively, the tool may be an elongated externally tooth broach for cutting slots to form internal teeth in a generally circular array in an opening in a workpiece.
In either case, the teeth constituting a longitudinally extending series may be straight and parallel to the axis of the broach, to form spur teeth, or arranged in a helix to form helical teeth on the workpiece.
Also, in either case, the teeth in each longitudinally extending series are of progressively stepped height so as to take a series of cuts to form the corresponding slot.
It is an advantage of broaching that the workpiece is positioned laterally, at least in the final phase of the operation, by the broaching tool itself. This results in producing workpieces whose accuracy is determined essentially by the accuracy of the broach. Since each broach produces hundreds or thousands of workpieces, it is practical to form broaches with extreme accuracy, thus, producing gears of comparable accuracy.
This of course requires that at least some broach teeth preferably spaced more or less uniformly around the circular array of slots being cut, remain at all times in cutting contact with the workpiece.
In the case of relatively thick gears, this requirement presents no difficulty and each circular set or array of cutting teeth may provide a tooth for each of the slots to be cut. However, if the thickness of a workpiece is less than the axial spacing between adjacent sets of teeth, it will be apparent that there is an interval between the cutting and guiding action of successive sets of teeth when guiding action is lost. On the other hand, if the axial spacing between adjacent sets of teeth is reduced enough to maintain continuous cutting and guiding action, the space between teeth in each longitudinal series may not be sufficient for adequate chip clearance.
In accordance with the present invention, each circumferentially extending set of teeth (formed on a separate ring in the specific embodiment disclosed) comprises alternated groups of teeth having one or more teeth therein, and gaps between adjacent groups of teeth, which gaps are devoid of teeth. In adjacent rings or sets of teeth, a group of teeth is in longitudinal alignment with a gap.
With this arrangement, axial spacing between adjacent rings or sets of teeth may be small enough to maintain continuous cutting action at all times for proper guiding, and there will also be sufficient space between adjacent teeth in each longitudinal series for clearing chips because of the gaps.
While it is possible to provide the teeth as described on a solid broach, or an elongate sleeve (in the case of an externally toothed broach), or in pot broach rings of substantial axial extent, it is preferred to provide the teeth on relatively thin flat rings, so that each tooth may have an axial dimension such as to provide a plurality of resharpenings by face grinding, the gap between successive cutting teeth in each longitudinally extending series providing space for chip disposal.
Also, while it is possible to omit alternate teeth on each ring, it is preferred to provide tooth portions comprising a group of several adjacent teeth together with gaps accommodating the teeth of the aligned group.
For illustration, specific disclosure herein is made of a spur pot broach comprising a multiplicity of thin rings, each of which comprises circumferentially extending groups of several teeth separated to provide gaps of corresponding circumferential extent.
The holder in which the rings may be assembled to form a pot broach is disclosed in Bistrick, et al, U.S. Pat. No. 4,065,222.